MAP

Saturday, 25 October 2014

Here’s one reason libraries hang on to old science
journals: A paper from an experiment conducted 32 years ago may shed light on
the nature ofdark
matter, the mysterious stuff
whose gravity appears to keep the galaxies from flying apart. The old data put
a crimp in the newfangled concept of a "dark photon" and suggest that
a simple bargain-basement experiment could put the idea to the test.

No one really knows what dark matter is. Since the
1980s, theorists' best hunch has been that it consists of so-called weakly
interacting massive particles, or WIMPs. If they exist, WIMPs would have a mass
between one and 1000 times that of a proton. They would interact only through
the feeble weak nuclear force—one of two forces of nature that ordinarily flex
their muscle only within the atomic nucleus—and could disappear only by
colliding and annihilating one another. So if the infant universe cooked up
lots of WIMPs, enough of them would naturally survive to produce the right
amount of dark matter today. But physicists
have yet to spot WIMPs, which every now and then should ping off atomic
nuclei in sensitive detectors and send them flying.

More recently, theorists have explored other ideas,
such as self-interacting dark matter. This would consist of a particle, known
as a χ (pronounced chi), with a mass between 1/1000 and one times that of the
proton. Those particles would interact with one another through a force like
the electromagnetic force, which produces light. That force would be conveyed
by a massive particle called a dark photon—a dark matter version of a particle
of light—that might "mix" slightly with the ordinary ones. So with
some small probability, a dark photon might interact with ordinary charged
particles such as electrons and atomic nuclei—just as ordinary photons do.

Self-interacting dark matter has attractive properties.
In particular, a dark photon could also explain a particle physics puzzle. A
particle called the muon appears to be very
slightly more magnetic than theory predicts, and that discrepancy could be
resolved if the muon interacts with dark
photons lurking in the vacuum. However, χs and dark photons would be hard to detect with WIMP
detectors; with their low masses, they couldn't whack a nucleus hard enough to
create a signal.

But archival data already rule out dark photons with
certain combinations of properties, argues RouvenEssig, a theoretical
physicist at Stony Brook University in New York, and his colleagues. The data
come from E137, a "beam dump" experiment that ran from 1980 to 1982
at SLAC National Accelerator Laboratory in Menlo Park, California. In the
experiment, physicists slammed a beam of high-energy electrons, left over from
other experiments, into an aluminum target to see what would come out.
Researchers placed a detector 383 meters behind the target, on the other side
of a sandstone hill 179 meters thick that blocked any ordinary particles. They
then looked for hypothetical particles called axions, which would have pierced the earth and reached the
detector—and saw none.

But electrons hitting the target should also have
produced a beam of high-energy χs. A χ could have traversed the hill and interacted with
an electron in the detector through a dark photon, blasting it into motion. The
fact that E137 saw no recoiling electrons enabled Essig and his colleagues
to nix
some possible combinations of the dark photon's mass and the strength of its
mixing with ordinary
photons, as they report this week in Physical Review Letters. The results do not
prove that the dark photon cannot exist at all, but they do put limits on its
possible properties.

Other physicists have used archival data to test new
dark matter theories. Last year, Philip Schuster, a theorist at the Perimeter
Institute for Theoretical Physics in Waterloo, Canada, and a colleague used the
result from another beam dump experiment at SLAC that ran in 1994 and 1995 to
probe self-interacting dark matter. But the millicharge, or mQ, experiment was sensitive to χs sending atomic nuclei
flying and set somewhat looser limits. "The electron-recoil limit looks a
little better," Schuster says.

With certain assumptions, the analysis disfavors a dark
photon with the properties needed to explain the muon's magnetism. But those assumptions could be loosened and
the idea more thoroughly tested with a new experiment, Schuster says. He and
roughly 80 other physicists hope to build a new beam dump experiment called
BDX, which would look at 100 times as many events as E137 did. They have
submitted a letter of intent to the Thomas Jefferson National Accelerator
Facility in Newport News, Virginia, although the experiment could be staged
elsewhere.

Compared with some particle physics experiments, BDX
would be small and cheap, says Marco Battaglieri of Italy's National Institute for Nuclear Physics in
Genoa and co-spokesman for the BDX team. "We are not talking about
thousands of tons of detector," he says. "We are talking about a
1-ton detector." BDX would cost a few million dollars, Battaglieri says.

The study also suggests it's not so easy to dream up
models of dark matter that don't run afoul of data already taken, Schuster
says: "All of this has to be done in a very tight straitjacket."